Ferrous iron can occur as an unwanted constituent in a number of potable, process, or wastewater streams. For example, ferrous iron in parts-per-million concentrations is present in many potable water systems, and can cause a number of problems for the end-user. Such problems range from the staining of laundry and plumbing fixtures by the subsequent slow formation of ferric iron to the aggravation of health problems in susceptible individuals. In such cases it is necessary to remove the iron from the water prior to distribution to the end-user if the aforementioned problems are to be avoided. This may be accomplished by oxidizing the ferrous iron to ferric iron at a pH greater than 5, and removing the precipitated ferric iron by filtration. In other liquid streams, such as those used to recover copper and uranium by leaching techniques, the presence of ferrous iron above a certain concentration can reduce the efficiency of metal extraction during the leaching process. The oxidation of ferrous iron in these applications is even more demanding since the low pH of the hydrometallurgical solutions, typically less than pH 5, strongly inhibits the oxidation reaction.
Past practices for the oxidation of ferrous iron in the aforesaid applications have relied either on the direct chemical oxidation of ferrous iron by the use of oxidants, such as chlorate or permanganate, or the use of materials, such as activated carbon, which are capable of catalyzing the oxidation of ferrous iron in the presence of oxygen. The oxidation of ferrous iron by means of chemical oxidants is often expensive and inconvenient, and usually requires continuous oxidant addition and monitoring of oxidant dose rates as a function of metal concentration. Moreover, the oxidants used are often expensive and toxic per se. If present in excess, they may present secondary health or environmental concerns. Even if present in stoichiometric amounts, the products of their reduction can also be toxic or can accumulate to unacceptable levels upon the repeated cycling of the liquid media containing them. In contrast to the use of chemical oxidants, the catalytic oxidation of ferrous iron by oxygen in the presence of activated carbon is inherently more convenient, since it requires only the addition of oxygen which can be cheaply, safely, and conveniently supplied in the form of air. For these reasons the use of activated carbon may be preferred practice if it can be made to be economically viable.
Although certain activated carbons have long been known for their ability to catalytically oxidize ferrous iron into ferric iron, the rate and extent to which such carbons can effect this conversion has often not been sufficient to allow their widespread use per se in such applications. In fact, some carbons are virtually inactive catalytically in those regions of low pH where ferrous iron is relatively stable to oxygen. To obtain satisfactory performance in applications that have commercial significance, it has usually been necessary to treat the carbons further to impart certain properties which enhance their ability to catalyze the oxidation reaction.
Some of these carbon treatments have involved the impregnation of an activated carbon with a metal which will catalyze the oxidation of ferrous iron. In such cases the carbon acts primarily as a support for the metal catalyst and does not contribute directly to the catalysis. For example, Lisitsyn et at. (React. Kinet. Catal. Lett. 49(1), 119 (1993)) have described the use of platinum catalysts impregnated on activated carbon to enhance the oxidation of ferrous iron by oxygen. However, the use of platinum is prohibitively expensive in many applications. Moreover, the presence of metals on the carbon invariably increases the inherent toxic hazard of the carbon and may present disposal problems when the carbon reaches the end of its usefulness. For metal impregnants other than the platinum group metals, there is also the potential of metal dissolution and leaching when used in low pH applications.
In other carbon treatments, properties are imparted into the activated carbon itself to enhance its ability to catalyze the oxidation of ferrous iron. For example, Culligan Corporation has described a process whereby activated carbon is treated with hypochlorite solution under ambient conditions to enhance its effectiveness for the oxidation and removal of ferrous iron from potable water streams (U.S. Pat. No. 4,534,867). Other post-treatments of activated carbon have involved the exposure of activated carbon to nitrogen-containing compounds at high temperatures. For example, Naito et at. (Nippon Kagaku Kaishi 4, 467 (1979)) have described a process whereby activated carbon is coated with a nitrogen-containing compound such as hexamine, ammonium chloride, urea, or melamine, and then calcined at high temperatures, typically 900.degree. C. The amount of nitrogen-containing compound employed is typically high, e.g. 20 wt % versus the weight of activated carbon. Treatment of an activated carbon with ammonia at high temperatures has also been found to be effective in enhancing the ability of an activated carbon to oxidize ferrous iron.
In a recent Russian patent (SU 1560592 A1), Konopleva et at. have described a process wherein a nitrogen-containing SKN- or SKAN-type carbon is used to enhance the oxidation of ferrous iron. Activated carbons of this type are typically produced by carbonizing and activating a nitrogen-rich synthetic polymer. For example, a vinyl-pyridine resin is used to produce SKN-type activated carbons (cf. I.A. Tarkovskaya et at., Soviet Progress in Chemistry 49, 18 (1983)), while SKAN-type activated carbons are produced from an acrylonitrile-divinylbenzene resin (V.A. Platonov et al., Khim. Tekhnol. (Kiev) 6, 56 (1991)).
Although reasonably effective in enhancing the ability of an activated carbon to catalyze the oxidation of ferrous iron, all of the prior art carbon-based processes have certain disadvantages which limit their overall economic utility. For example, the use of a synthetic nitrogen-containing resin or polymer as a starting material is inherently expensive, and invariably involves the generation of large amounts of hazardous cyanides during carbonization and activation. Activated carbons produced from cheaper, naturally-occurring, nitrogen-poor feedstocks do not have the catalytic activity necessary for the broad range of conditions often found in iron removal applications, low pH conditions in particular. Where such activated carbons have been post-treated at high temperatures with nitrogen-containing compounds to improve their catalytic activity, the processes employed for their beneficiation have been inherently expensive and hazardous, yielding products of variable quality and marginal economic utility. Such processes are expensive because they employ a finished high-temperature char, such as an activated carbon, as the primary feedstock. Such feedstocks are relatively inert chemically and require large quantities of reagents, high carbon losses, and/or significant departures from standard activated carbon production practices to effect significant gains in the catalytic activity of the final product. Additionally, these processes can be hazardous because they often employ hazardous reagents, such as caustic hypochlorite, or generate significant amounts of toxic byproducts, such as cyanide or nitrogen oxides, during processing.
Accordingly, it is the object of the present invention to provide a process for the oxidation of ferrous iron in liquid media which is economical, convenient, effective, and environmentally satisfactory. It is further the object of the present invention to employ a carbon for this process which is made directly from an inexpensive and abundant nitrogen-poor starting material, such as a bituminous coal, and to limit the use of agents responsible for imparting the catalytic properties to the starting material by performing the essential treatment steps during the transition of the starting material into the final product. It is a further object of the invention to utilize carbon treatment steps which include low-temperature carbonization and oxidation of the starting material, preferably by inexpensive, abundant, and relatively non-toxic oxidants, and exposure of the carbonized, oxidized, low-temperature char to small amounts of inexpensive, abundant and relatively non-toxic nitrogen-containing compounds before or during, but not after, the initial calcination and condensation of the carbon structure. It is generally the object of the invention to provide carbon treatments that are highly compatible with current processes for manufacturing activated carbons, and can be carded out with minimal departures from conventional practice.